1. Field of the Invention
The present invention is directed to a method and material for protecting bonding sites on a module during a solder dipping operation, and more particularly relates to an improved strippable solder mask for such bonding sites.
2. Prior Art
In the manufacture of pin electronics modules, for example in a multilayer ceramic hybrid configuration, there are gold thermocompression bonding sites to which leads are connected to provide the interconnection between the modules and the external world. In the process of producing the module, a solder dipping operation occurs for the purpose of connecting the pins to the module circuitry. For some substrates the chip is then connected to the circuitry via thermocompression bonding of wires from the chip to gold pads on the circuitry. During the solder dipping operation, it is important that the gold thermocompression bonding sites on the module be protected in a manner such that after the solder dipping operation, the gold bonding sites can be cleaned and used to provide the above mentioned interconnections. Conventional stop resist materials, such as positive and negative photoresists or sodium silicate, have been used to mask the bonding sites. However, with such materials the bonding sites have often experienced problems either due to poor solder protection or alternatively to incomplete strippability prior to the wire bonding that subsequently occurs. When the poor solder protection occurs, some of the solder penetrates to the bonding sites which adversely affects the subsequent wire bonding to the site. Similarly, when the mask material does not completely strip away prior to wire bonding, the wire bond will have a diminished bonding strength.